When starting up an optical transmission apparatus and node for the first time, work is carried out to configure individual plugin units and check device operation.
For example, in an optical amp, gain is configured by utilizing amplified spontaneous emission (ASE). This involves causing ASE to be produced from an optical amp at a node positioned upstream along the optical fiber line, and then causing the ASE to be input into an optical amp at a node positioned downstream. Subsequently, the gain is configured such that the optical output level from the downstream amp reaches a desired level.
Related technology for starting up an apparatus by utilizing ASE is proposed in, for example, Japanese Unexamined Patent Application Publication No. 2004-23437.
Besides configuring the gain in the optical amp, another important step when first starting up a system that conducts optical transmission using wavelength-division multiplexing (WDM) is checking whether or not optical signals are being output at normal levels after wavelength separation in the apparatus. However, in the related art, it has been difficult to precisely check the normality of optical signal levels after wavelength separation utilizing ASE.
FIG. 11 illustrates a WDM transmission apparatus. FIG. 11 illustrates the portion of a WDM transmission apparatus 5 configured to receive and separate a WDM optical signal multiplexed with light having a plurality of respectively different wavelengths. The WDM transmission apparatus 5 includes the following plugin units: an optical preamp unit 51, and a wavelength separation unit 52. The optical preamp unit 51 includes a preamp 51a and a coupler 51b. Meanwhile, the wavelength separation unit 52 includes a DMUX 52a, couplers 52b-1 to 52b-n, and photodiodes (PD) 52c-1 to 52c-n. 
The preamp 51a amplifies and outputs a received WDM signal flowing in from upstream along an optical fiber line F. The amplified WDM optical signal is split into two parts by the coupler 51b, with one part being provided to a post-processor, and the other part being provided to the wavelength separation unit 52.
The DMUX 52a separates the received WDM optical signal into n wavelengths. Each of the couplers 52b-1 to 52b-n then splits the optical signal for one of the wavelengths into two parts, with one part being provided to one of the PDs 52c-1 to 52c-n, and the other part being directed to a tributary and dropped. Each of the PDs 52c-1 to 52c-n generates an electrical signal by O/E converting the received optical signal for one of the wavelengths. The generated electrical signals are then provided to a predetermined processor.
When starting up the WDM transmission apparatus 5 herein for the first time, the configuring and checking work is conducted individually for the respective plugin units (i.e., the optical preamp unit 51 and the wavelength separation unit 52).
Work performed with respect to the optical preamp unit 51 when starting up the apparatus may involve, for example, causing ASE provided from an upstream node and flowing along the optical fiber line F to be input into the preamp 51a, and configuring the gain in the preamp 51a. 
Meanwhile, work performed with respect to the wavelength separation unit 52 when starting up the apparatus may involve, for example, checking whether or not the optical signals are being output (or dropped) at normal levels after the wavelength separation in the DMUX 52a. In this case, it can be checked whether or not the optical signals at the respective wavelengths are at normal levels by examining the electrical signal levels after O/E conversion in each of the PDs 52c-1 to 52c-n. 
In order to precisely check the normality of the optical signal levels after wavelength separation, it is preferable for the optical signal input into the DMUX 52a to have an optical power that is close to the power of a WDM optical signal received during normal operation. In other words, it is preferable for the optical signal input into the DMUX 52a to have an optical power in the WDM optical signal wavelength band that is nearly identical to the optical power when receiving a WDM optical signal during normal operation.
When inputting ASE into the wavelength separation unit 52, the ASE provided from an upstream node travels along the optical fiber line F, and thus its optical power is extremely low upon arrival. For this reason, in order to make the optical power close to the optical power when receiving a WDM optical signal during normal operation, the low-power ASE is amplified to high-power ASE in the preamp 51a. 
However, if the gain is increased to raise the low-power ASE flowing in along the optical fiber line F to an optical power nearly equal to the optical power when receiving a WDM optical signal during normal operation, the shape of the wavelength profile becomes significantly sloped.
FIG. 12 illustrates such sloping being produced in the shape of the wavelength profile. In FIG. 12, the horizontal axis expresses the optical power (in dBm), while the vertical axis expresses the wavelength (in nm). If a standard erbium-doped fiber amplifier (EDFA) is used as the optical amp, then as the gain of the EDFA is raised, population inversion also rises. It has been established that in such a state with high population inversion, the optical power with respect to the wavelength slope down and to the right in the region corresponding to the WDM optical signal wavelength band, such as near the range from 1530 nm to 1570 nm. (In contrast, if the gain is lowered to create a low population inversion state, then the optical power with respect to the wavelength slope up and to the right.) Meanwhile, it has been established that flatness is obtained when the population inversion is kept to approximately 70%.
FIG. 13 illustrates a WDM transmission apparatus. FIG. 13 illustrates the outputting of ASE produced by a sloped wavelength profile. In the wavelength profile pr1 at the input stage of the preamp 51a, the ASE 2 provided from an upstream node has a flat optical power in the WDM optical signal wavelength band (i.e., the optical power is uniform at those wavelengths), but the optical power is also very low overall.
Given such ASE 2, if the gain in the preamp 51a is then increased to amplify the ASE 2 to an optical power close to the optical power when receiving a WDM optical signal during normal operation, the wavelength profile of the ASE 2a output from the preamp 51a loses much of its flatness. In other words, as illustrated in the wavelength profile pr2, the resulting ASE is high-power, but the portion that was flat in the wavelength profile pr1 now slopes down and to the right.
If the ASE 2a having such a wavelength profile pr2 is input into the DMUX 52a, then even if the optical signal levels after wavelength separation are monitored, differences in the optical levels at respective wavelengths are already produced before wavelength separation. For this reason, strict checking of the normality of the optical signal levels after wavelength separation becomes problematic.
Meanwhile, it is also conceivable to check the normality of optical signal levels after wavelength separation by forgoing use of the ASE 2 provided from the upstream node, putting the preamp 51a into an input-less state, and inputting ASE produced by the preamp 51a itself into the DMUX 52a. 
In this case, it is still preferable for the ASE input into the DMUX 52a to have an optical power close to the optical power when receiving a WDM optical signal during normal operation. Thus, the preamp 51a amplifies the self-produced ASE to a predetermined level before output.
However, outputting ASE with an optical power close to the incoming level of a WDM optical signal during normal operation from a preamp in an input-less state is dependent upon the amplification performance of the preamp 51a, and thus imposes restrictions on the manufacturing specifications of the preamp 51a itself. This method is not readily applicable to an arbitrary preamp.
Even if it were hypothetically possible to increase gain and output ASE that has been amplified to a predetermined level from the preamp 51a in an input-less state without imposing restrictions on the manufacturing specifications of the optical preamp unit 51a itself, a large gain still be set in the preamp 51a, and thus the ASE output from the preamp 51a have a wavelength profile that has lost some of its flatness, like the above wavelength profile pr2. Consequently, it is still difficult to strictly check the normality of optical signal levels after wavelength separation, even with the method of producing ASE from the preamp 51a itself.
As described above, in a WDM transmission apparatus of the related art, it is difficult to produce ASE having a flat and high-output wavelength profile, and thus it is also difficult to precisely check the normality of optical signal levels after wavelength separation.
Being devised in light of such points, one object of the preprovided invention is to provide an optical transmission apparatus and an optical signal level checking method that enable the normal operation of apparatus functions to be checked with high precision by utilizing ASE.